Selectivity of herbicides applied in the post-emergence of industrial hemp ( <i>Cannabis sativa</i> L.)

The objective of this study was to evaluate the selectivity of four post-emergence herbicides in the cultivation of industrial hemp. Hemp was grown in 3.5 L pots with inert substrate in the absence of weeds throughout the crop cycle. The experimental design adopted was a randomized, complete block design with five treatments and four replications. The herbicides atrazine (2.500 g a.i./ha), clethodim (96 g a.i./ha), fomesafen (250 g a.i./ha), and nicosulfuron (54 g a.i./ha) were applied when the plants were approximately 35 days after cutting. Visual assessments of herbicide-induced injury symptoms were conducted, along with measurements of the number of leaves, plant height, stem diameter, and total aerial part dry mass. When nicosulfuron was applied after emergence, it caused 60.0% damage 21 days after application (DAA), which changed the number of leaves, height, stem diameter, and total aerial part dry mass of hemp plants in a big way. Fomesafen caused 42.5% injury at 5 DAA, and atrazine caused 42.5% injury at 21 DAA, which may compromise the final yield. Therefore, it is recommended to use only clethodim, a selective herbicide for broadleaf plants such as hemp, as it did not cause significant injuries during the evaluation period.